[llvm] [SCEV,LAA] Introduce scoped SCEV, use in LAA computations (WIP). (PR #90742)

[Nikita Popov via llvm-commits]

nikic wrote:

The current handling of SCEV flags is a pretty general issue, and the approach used here will only be usable in limited circumstances -- ideally, we would want to be able to preserve flags when going from IR to SCEV, but I don't think that the scope-based approach here will enable that (or at least, not to a significant degree and not without complications).

The general approach I would consider is to replace our current `const SCEV *` pointers with `SCEVUse`, where the low bits are used to store no-wrap flags for a specific use of the expression.

https://github.com/llvm/llvm-project/pull/90742